The present invention relates to an additive for a fuel to neutralize sulfur dioxide and/or sulfur trioxide in the exhaust gases of internal combustion engines powered by the fuel, as well as a method for the neutralization of sulfur dioxide and/or sulfur trioxide in such internal combustion engines.
This application claims the priority of German application no. 198 18 536.7, filed Apr. 24, 1998, the disclosure of which is expressly incorporated by reference herein.
The exhaust gases of internal combustion engines always contain sulfur in the form of sulfur dioxide and/or sulfur trioxide. Consequently, an undesired sulfuric acid exhaust gas can form. Also, these sulfur compounds interfere with the catalytic exhaust gas cleaning of Otto-cycle and Diesel engines. The elimination of these sulfur oxides from the exhaust gases is therefore desirable.
A special problem is encountered in connection with so-called NO.sub.x accumulator catalysts which are seen as a possible alternative to nitrogen oxide elimination in internal combustion engines, especially in lean-mix and Otto-cycle engines. The NO.sub.x accumulation medium consists substantially of barium and/or strontium compounds which are mixed with aluminum oxide. However, due to the manner of operation of this catalyst and the material used for the accumulation of NO.sub.x, problems with the sulfur accumulation in the exhaust gas can occur. This is the result of the following considerations: the lambda number is known as a measure of the ratio of the amount of air or oxygen introduced into the combustion chamber of an internal combustion engine that is theoretically necessary for complete combustion. In internal combustion engines, the lambda number indicates the ratio of the total oxygen to the amount of oxygen that is needed for the complete combustion of the air and fuel mixture in the cylinder. Internal combustion engines with exhaust cleaning by controlled three-way catalysts must be operated at lambda number 1 and, if NO.sub.x -accumulating catalysts are used, at lambda number 1 or greater, so as to permit optimum exhaust gas cleaning. In the case of a high air-to-fuel ratio, i.e., in an oxidizing atmosphere (lambda.gtoreq.1), however, two reactions compete with one another in the NO.sub.x accumulator-catalyst:
1. The nitrogen oxide NO.sub.2 reacts with the barium carbonate in the desired absorption reaction to form barium nitrate: EQU BaCO.sub.3 +2NO.sub.2 +1/2O.sub.2.fwdarw.Ba(NO.sub.3).sub.2 +CO.sub.2 (.lambda..gtoreq.1) PA1 2. The sulfur reacts as sulfur trioxide with barium carbonate to form barium sulfate: EQU BaCO.sub.3 +SO.sub.3.fwdarw.BaSO.sub.4 +CO.sub.2 (.lambda..gtoreq.1)
Therefore the absorption potential for nitrogen oxides is lost, and the catalyst becomes contaminated.
In addition, the reaction preferentially leads to the very stable barium sulfate if sulfur trioxide is present in the exhaust gas. In the combustion of sulfur from the fuel and the oil only sulfur dioxide is formed. However where there are air:fuel ratios (lambda.gtoreq.1), the sulfur dioxide is oxidized to sulfur trioxide. This reaction is retarded with the increasing temperature of the internal combustion engine.
This differs from Otto-cycle engines in passenger vehicles and Diesel engines of passenger vehicles and utility vehicles, where it has been shown that the formation of sulfates virtually cannot be suppressed. This indicates that, even at higher temperatures, sufficient sulfur trioxide is formed. Since most of the sulfur originates from the fuel (5-700 ppm depending on the quality) the life of NO.sub.x accumulator catalysts is thus limited.
In the firing of power plants and waste incinerators it is known to feed into the fire sulfur-binding substances such as alkali and alkaline earth carbonates, as well as alkali and alkaline earth oxides (DE 33 06 795 C1, DE 32 34 315 A1, DE 38 40 212 C2). It is furthermore known to add metal salts of organic acids to such fires in order to reduce the emission of sulfur dioxide and sulfur trioxide (JP 50-117 805 A, JP 54-081 536 A) . However, these methods are not transferrable to internal combustion engines.
The present invention relates to the problem of providing an additive and a method that will permit reliable desulfurization of the exhaust gases of internal combustion engines.
The present invention accomplishes this by adding an additive comprising one or more metal compounds that are soluble in fuel and which in the combustion of the fuel form stable sulfates which are present in solid form in the exhaust gas.
Thus, according to the invention, sulfur dioxide and sulfur trioxide are bound in the form of stable sulfates which form in the combustion and whereby they are discharged in the form of solid particles together with the exhaust gas from the internal combustion engine. The metal sulfate particles can then be deposited themselves in the exhaust tract, for example in the muffler, or they can be blown out of the tail pipe. In this manner a reliable desulfurization of the exhaust gases is achieved. Moreover, the contamination of the NO.sub.x catalytic accumulator is avoided.